The Mozilla Links blog points out a few about:config tweaks that Firefox 3.6 users might be interested in, especially if the new version handles things a bit differently than you're used to, and you don't want to spend time getting used to the new scheme. To set new tab behavior back to its old ways—opening in the right-most slot—enter about:config into your address bar, click that you'll be careful, then enter browser.tabs.insertRelatedAfterCurrent into the search bar. Set that value to "False," and your new tabs will open where they used to.